1. Field of the Invention
The present invention relates to polymer blends, to methods of making such blends, to products made from such blends, and to methods of molding. In another aspect, the present invention relates to polymer blends having improved environmental stress cracking or environmental fatigue resistance, to methods of making such blends, to products made from such blends, and to methods of molding such products. In even another aspect, the present invention relates to blends of copolymers of olefins/and ethylenically unsaturated carboxylic ester monomers, to methods of making such blends, to products made therefrom, and to methods of molding such products. In still another aspect, the present invention relates to blends of copolymers of olefins/and ethylenically unsaturated carboxylic ester monomers having improved environmental stress cracking or environmental fatigue resistance, to methods of making such blends, to products made therefrom, and to methods of molding such products.
2. Description of the Related Art
Many applications require a material which exhibits a resistance to stress cracking and fatigue failure during exposure to an aggressive chemical environment. Stress cracking or environmental stress cracking (ESC) is the brittle failure of plastic parts when simultaneously subjected to mechanical stress and chemical exposure. In a similar fashion, environmental fatigue (EF) is the failure or cracking or a part when simultaneously subjected to dynamic mechanical stress and chemical exposure. Insufficient environmental stress cracking or environmental fatigue resistance leads to greatly shortened service life of a part.
Ethylene/vinyl acetate copolymers are commonly injection molded into products utilized in applications requiring good environmental stress cracking or environmental fatigue resistance.
One common way of obtaining an ethylene/vinyl acetate copolymer with good environmental stress cracking or environmental fatigue resistance is to select a high molecular weight viscous resin especially a resin with a narrow molecular weight distribution. Unfortunately, some processes cannot utilize such a viscous resin. For example, in a process such as injection molding, small intricate, mold passages can pose a problem for such a viscous resin. Consequently, this approach of utilizing a high molecular viscous resin is not useful when fabricating a part by injection molding.
There are numerous patents disclosing polymers having improved environmental stress crack resistance.
U.S. Pat. No. 3,261,889, issued Jul. 19, 1966 to van't Wout, discloses a polyethylene-containing material having an increased resistance to environmental stress cracking. The increased resistance is provided by the addition of an atactic copolymer of ethylene with propylene and/or butene-1, and/or by the addition of an atactic copolymer of ethylene with propylene and 1-10% by weight of a diene containing 5-20 carbon atoms.
U.S. Pat. No. 3,382,298, issued May 7, 1968 to Larsen et al., discloses a blend of low density polyethylene and either a polyvinyl ester or polyvinyl acetal. The polyethylenes utilized have a melt index from about 0.01 to about 50 decigrams/minute, and the polyvinyl esters utilized have molecular weight from about 7,000 to 140,000.
U.S. Pat. No. 3,410,928, issued Nov. 12, 1968 to Baum, discloses blends of olefin high polymers and ethylene/acrylic acid copolymers which exhibit greatly improved adhesivity, printability, grease resistance, and stress crack resistance compared with olefin polymers alone. The olefin polymers have a molecular weight of at least 10,000 or a melt index of about 100 or less.
U.S. Pat. No. 3,485,783, issued Dec. 23, 1969, to Kehe, discloses a hot melt adhesive composition that is a blend of an ethylene/alkyl acrylate copolymer, an ethylene/acrylic acid copolymer, and a polymerized rosin ester.
U.S. Pat. No. 3,533,976, issued Oct. 13, 1970, to Eldman, discloses polyolefin blends with superior stress crack resistance comprising polyethylene resin, an ethylene vinyl acetate copolymer, a synthetic paraffin wax, carbon black and an antioxidant. The blend comprises 76 to 92 weight percent polyethylene, and 7 to 15 weight percent ethylene vinyl acetate.
U.S. Pat. No. 3,663,663, issued May 16, 1972 to Bernie et al., discloses polyolefin-ethylene/ester copolymer blend compositions having improved stress cracking properties, especially in the presence of stress crack promoting detergents. The blends comprise a high molecular weight polyethylene having a melt index in the range of about 0.01 to about 10.0, and a copolymer of ethylene and an ester comonomer having a molecular weight below 15,000.
U.S. Pat. No. 3,770,852, issued Nov. 6, 1973 to Hager et al., discloses polyolefin blends having broad polymodal molecular weight distribution and improved environmental stress crack resistance, which are prepared by polymerizing an olefin or a mixture of ethylene and vinyl acetate in a first reaction region, passing the resulting reaction product to a second reaction region into which a second olefin gas stream is fed and polymerized and blending the polymerization product from the second reaction region with a single reactor ethylene-vinyl acetate copolymer. The dual reactor copolymers have a vinyl acetate content up to about 6 percent. The single reactor copolymer contains from 0 to about 70 weight percent vinyl acetate, and has a melt index from about 1 to 5,000. The '852 patent teaches that there is no advantage in the blend having more than ten percent of the single reactor ethylene-vinyl acetate copolymer.
U.S. Pat. No. 3,808,047, issued Apr. 30, 1974 to McAda, discloses a polyethylene blend having improved stress cracking resistance comprising a high molecular weight polyethylene having a melt index from 0.01 to 10 decigrams/minute, and a copolymer of ethylene and an ester comonomer, with the copolymer having a molecular weight below 15,000 and from 20 to 65 weight percent ester comonomer moiety. The blend comprises 85 to 95 weight percent of the high molecular weight percent polyethylene, and from 5 to 15 weight percent of the low molecular weight copolymer. The method for preparing the blend includes introducing the copolymer into the polyethylene in the molten state as it passes from the polymerization zone.
U.S. Pat. No. 4,312,918, issued Jan. 26, 1982 to Bostwick, discloses compositions of polyethylene and a copolymer of ethylene-alkyl acrylate having improved environmental stress cracking resistance.
However, in spite of these advancements in the prior art, there is still a need for an ethylene/vinyl acetate copolymer having improved stress cracking resistance.
There is another need in the art for a method of improving the stress cracking resistance of an ethylene/vinyl acetate copolymer.
These and other needs in the art will become apparent to those of skill in the art upon review of this specification, including its drawings and claims.